1. The Field of the Invention
This invention relates to a compound objective lens suitable for imaging objects of substantial radial dimension, at high numerical aperture where each of the lens elements is re-used such that only two distinct elements are employed in the objective. The compound lens is particularly well suited as an objective lens for a near infrared spectrometer, and a well optimized design for such a spectrometer is presented.
2. Background and Relevant Art
Compound objective lenses suitable for imaging objects of substantial radial extent with reference to the focal length of the lenses have been demonstrated in various imaging applications. In U.S. Pat. No. 3,994,576, Glatzel et al. discloses a five element objective lens system of the expanded Tessar type wherein two of the elements are cemented to each other. Similarly in U.S. Pat. No. 4,101,201, Tojyo et al. discloses a low magnification microscope objective comprising two components of four elements. Further in U.S. Pat. No. 4,105,291, Tsuji et al. discloses a large aperture zoom lens comprising four groups of elements, each group consisting of two or more distinct lenses. Additionally, in U.S. Pat. No. 4,123,144, Mandler et al. discloses a four member Gauss objective with a net of six distinct elements. In U.S. Pat. No. 4,556,295 Okudaira et al. discloses a telephoto lens comprising ten or eleven distinct elements. Similarly, in U.S. Pat. No. 4,726,669, Angenieux discloses a high numerical aperture objective lens with compound focusing comprising four components, some of which consist of multiple lens elements. Mercado in U.S. Pat. No. 5,000,548 discloses a microscope objective with four distinct lens elements, whereas, Kung, in U.S. Pat. No. 5,216,548, discloses a compound lens with four elements for photographing printed objects of large extent. Finally, Koizumi et al. in U.S. Pat. No. 6,069,750 discloses a wide-field eye-piece comprising four lens groups and containing five distinct elements.
A common feature of all the aforementioned disclosures is the multiplicity of distinct lens elements. The minimum number for all the disclosed designs is four. A review of all the designs also shows that when:                a. the object to be imaged has radial extent of greater than 0.25 of the focal length of the compound lens,        b. and when the numerical aperture exceeds 0.25,        c. and when the required resolution is less than about 0.01 of the focal length,that the complexity and number of distinct elements in the compound lens increases.        
The cost of a compound lens will scale with the number of components. Also, when molding technology is used to form the lenses, a distinct mold is required for each element. Mold costs for precision optics can also be prohibitive so it is desirable to create the minimum number consistent with obtaining the required optical performance.
Double Gauss lenses have been employed in imaging applications, an example of which is disclosed in Mandler. In its general form it consists of positive meniscus lenses on the outside and negative meniscus lenses on the inside and may have additional elements. In some double Gauss arrangements lens elements are duplicated or nearly duplicated to obtain some aberration cancellation due to symmetry. It has not, however, heretofore been possible to obtain high resolution, high numerical aperture imaging with large field of view with double Gauss arrangements having less than four distinct elements.